The present invention relates to an MR (magnetic resonance) imaging method and an MRI (magnetic resonance imaging) apparatus, and more particularly to an MR imaging method and an MRI apparatus preventing deterioration in NMR (nuclear magnetic resonance) signals due to a slice-leaned state.
FIG. 1 is a drawing illustrating an example of pulse sequence SQxe2x80x2 by a conventional spin echo (SE) method.
As shown in FIG. 1(a), first a 90xc2x0 RF pulse R for exciting a desired slice is applied to effect excitation, and then a 180xc2x0 RF pulse P for exciting the same slice is applied to invert the slice. Then, as shown in FIG. 1(b), a spin echo signal E51 is observed.
As shown in FIG. 1(c), when applying the aforementioned 90xc2x0 RF pulse R and when applying the aforementioned 180xc2x0 RF pulse P, a slice gradient Bg1 where the magnetic field intensity with respect to the position in the slice thickness direction varies at the rate of the inclination angle G1 is added.
As shown in FIG. 2(a), a profile Ar pertaining to the aforementioned 90xc2x0 RF pulse R takes on a shape relatively close to the ideal rectangular waveform indicated by dotted lines. On the other hand a profile Ap51 pertaining to the aforementioned 180xc2x0 RF pulse P takes on a shape whose two shoulders are rounded unlike the ideal rectangular form indicated by dotted lines. Incidentally, the half power width of each profile is supposed to be the excitation width xcfx84.
Therefore, as shown in FIG. 2(b), the slice profile Fxe2x80x2 determined by the product of the aforementioned profile Ar and the aforementioned profile AP51 takes on a rounded shape with both shoulders gently sloped, generating a so-called xe2x80x9cslice-leanedxe2x80x9d state.
However, if the aforementioned slice-leaned state arises, there will arise the problem that no NMR signal is obtained in the shadowed area U between the profile and the ideal rectangular form indicated by the dotted lines, resulting in a deterioration in the quality of MR images.
An object of the present invention is to provide an MR imaging method and an MRI apparatus preventing deterioration in NMR signals due to a slice-leaned state.
In its first aspect, the invention provides an MR imaging method for applying a 180xc2x0 RF pulse to a specimen after applying a 90xc2x0 RF pulse to it, characterized in that the excitation width pertaining to the aforementioned 180xc2x0 RF pulse is made broader than the excitation width pertaining to the aforementioned 90xc2x0 RF pulse.
According to the MR imaging method in the aforementioned first aspect, since the excitation width pertaining to the 180xc2x0 RF pulse is made broader than the excitation width pertaining to the 90xc2x0 RF pulse, the width of the profile pertaining to the 180xc2x0 RF pulse is expanded in the slice thickness direction.
As a result, it is made possible to restrain the slice profile from falling into a slice-leaned state and thereby to prevent NMR signals from deterioration.
As the slice thickness pertaining to the slice profile is restricted by the profile pertaining to the 90xc2x0 RF pulse, it will not expand excessively.
In its second aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width is expanded by making the inclination angle of the gradient magnetic field with respect to the position in the slice thickness direction at the time of applying the 180xc2x0 RF pulse smaller than the inclination angle of the gradient magnetic field at the time of applying the 90xc2x0 RF pulse.
According to the MR imaging method in the aforementioned second aspect, there is no need to alter the 180xc2x0 RF pulse because the excitation width is expanded by making the inclination angle of the gradient magnetic field smaller at the time of applying the 180xc2x0 RF pulse. This is particularly useful where widening of the band of the RF pulse frequency component is restricted.
In its third aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width is expanded by reducing the width of the aforementioned 180xc2x0 RF pulse in the time axis direction.
According to the MR imaging method in the aforementioned third aspect, the excitation width can be expanded even where the inclination angle of the gradient magnetic field with respect to the position in the slice thickness direction at the time of applying the 180xc2x0 RF pulse is equal to the inclination angle of the gradient magnetic field at the time of applying the 90xc2x0 RF pulse, because the frequency component band is made broader by reducing the width of the 180xc2x0 RF pulse in the time axis direction to expand the excitation width. Where it is to be used in combination with a reduction in the inclination angle of the gradient magnetic field at the time of applying the 180xc2x0 RF pulse, the difference from the inclination angle of the gradient magnetic field at the time of applying the 90xc2x0 RF pulse, and the control of the gradient magnetic field can be made easier and more precisely.
In its fourth aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that, where multi-slice imaging is to be accomplished by successively applying a plurality of 180xc2x0 RF pulses, the excitation width pertaining to the aforementioned 180xc2x0 RF pulse is made broader than the excitation width pertaining to the aforementioned 90xc2x0 RF pulse by not smaller than a 0.4-fold multiple but not greater than a 0.6-fold multiple of the slice spacing.
According to the MR imaging method in the aforementioned fourth aspect, the NMR signal intensity improving performance due to expansion of the excitation width can be sufficiently achieved because the lower limit of expanding the excitation width pertaining to the 180xc2x0 RF pulse is made a 0.4-fold multiple of the slice spacing. Furthermore, as the upper limit of expanding the excitation width pertaining to the 180xc2x0 RF pulse is made a 0.6-fold multiple of the slice spacing, interference between adjoining slice profiles can be reduced.
As a result, even where multi-slice imaging is to be carried out, it is possible to prevent NMR signals from deterioration by restraining the occurrence of a slice-leaned state.
In its fifth aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width pertaining to an inversion pulse to be applied first out of a pulse sequence is made broader than the excitation width pertaining to the 90xc2x0 RF pulse.
According to the MR imaging method in the aforementioned fifth aspect, even where a pulse sequence of an inversion recovery (IR) method, by which an inversion pulse which inverts the direction of the nuclear magnetization vector by 180xc2x0 is applied at the beginning of the sequence, NMR signals can be prevented from deterioration by restraining the occurrence of a slice-leaned state.
In its sixth aspect, the invention provides an MR imaging method of the above-described configuration, characterized in that the excitation width of the aforementioned 180xc2x0 RF pulse is expanded so that the whole part excited by the aforementioned 90xc2x0 RF pulse be excited by the aforementioned 180xc2x0 RF pulse.
According to the MR imaging method in the aforementioned sixth aspect, the best SNR can be obtained.
In its seventh aspect, the invention provides an MRI apparatus provided with a gradient magnetic field generating unit for generating a gradient magnetic field; an RF pulse transmitting unit for transmitting RF pulses; and an NMR signal receiving unit for receiving NMR signals; and the MRI apparatus being characterized in that it is provided with an excitation width adjusting unit for making the excitation width pertaining to a 180xc2x0 RF pulse broader than the excitation width pertaining to a 90xc2x0 RF pulse.
The MRI apparatus in the aforementioned seventh aspect can appropriately implement the MR imaging method according to the aforementioned first aspect.
In its eighth aspect, the invention provides an MRI apparatus of the above-described configuration, characterized in that the aforementioned excitation width adjusting unit so controls the operation of the aforementioned gradient magnetic field generating unit as to make the inclination angle of the gradient magnetic field with respect to the position in the slice thickness direction at the time of applying the 180xc2x0 RF pulse smaller than the inclination angle of the gradient magnetic field at the time of applying the 90xc2x0 RF pulse.
The MRI apparatus in the aforementioned eighth aspect can appropriately implement the MR imaging method according to the aforementioned second aspect.
In its ninth aspect, the invention provides an MRI apparatus of the above-described configuration, characterized in that the aforementioned excitation width adjusting unit so controls the operation of the aforementioned RF pulse transmitting unit as to reduce the width of the aforementioned 180xc2x0 RF pulse in the time axis direction.
The MRI apparatus in the aforementioned ninth aspect can appropriately implement the MR imaging method according to the aforementioned third aspect.
In its 10th aspect, the invention provides an MRI apparatus of the above-described configuration, characterized in that the aforementioned excitation width adjusting unit, where multi-slice imaging is to be accomplished by successively applying a plurality of 180xc2x0 RF pulses, makes the excitation width pertaining to the aforementioned 180xc2x0 RF pulse broader than the excitation width pertaining to the aforementioned 90xc2x0 RF pulse by not smaller than a 0.4-fold multiple but not greater than a 0.6-fold multiple of the slice spacing.
The MRI apparatus in the aforementioned 10th aspect can appropriately implement the MR imaging method according to the aforementioned fourth aspect.
In its 11th aspect, the invention provides an MRI apparatus of the above-described configuration, characterized in that the aforementioned excitation width adjusting unit makes the excitation width pertaining to an inversion pulse to be applied first out of a pulse sequence broader than the excitation width pertaining to the 90xc2x0 RF pulse.
The MRI apparatus in the aforementioned 11th aspect can appropriately implement the MR imaging method according to the aforementioned fifth aspect.
In its 12th aspect, the invention provides an MRI apparatus of the above-described configuration, characterized in that the aforementioned excitation width adjusting unit expands the excitation width of the aforementioned 180xc2x0 RF pulse so that the whole part excited by the aforementioned 90xc2x0 RF pulse be excited by the aforementioned 180xc2x0 RF pulse.
The MRI apparatus in the aforementioned 12th aspect can appropriately implement the MR imaging method according to the aforementioned sixth aspect.
Therefore, The MR imaging method and the MRI apparatus according to the present invention makes it possible to obtain a slice profile closed to the ideal rectangular form and to generate MR images of high picture quality by, after exciting a prescribed excitation width with a 90xc2x0 RF pulse, exciting a broader range than the excitation width with a 180xc2x0 RF pulse.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.